1. Field of the Invention
The present invention relates to a power supply voltage controlling circuit for multiple power supplies and a semiconductor integrated circuit with the power supply voltage controlling circuit.
2. Background Art
There are power supply voltage controlling circuits (power supply voltage regulators) which lower the power supply voltage before supplying the voltage to a logic circuit or the like.
The input voltage of such conventional power supply voltage controlling circuits is set higher in order to allow a margin to derive a stable output voltage from the input voltage that varies significantly. Because of this input voltage margin, the power loss (=((input voltage)−(output voltage))*(input current)) inevitably increases if the input voltage of the power supply voltage controlling circuit is supplied from a single power supply system.
Furthermore, recently, it has become a common practice to lower the set value of the output voltage of the power supply voltage regulator to reduce the power consumption in a situation where the operation can be slowed down. In that situation, if the output voltage is lowered while a high input voltage is used, a considerable power loss occurs.
Thus, some of conventional power supply voltage controlling circuits have a problem that the power loss cannot be suppressed while allowing an input voltage margin to accommodate variations in input voltage, set voltage value or load current.
A conventional power supply voltage controlling circuit has a first power supply circuit that converts a voltage from a direct-current power supply into a first voltage and outputs the first voltage to an output terminal and a second power supply circuit that converts the voltage from the direct-current power supply into a second voltage and outputs the second voltage to the output terminal (see Japanese Patent Laid-Open Publication No. 2004-62331, for example).
This power supply voltage controlling circuit switches between the first power supply circuit and the second power supply circuit, which are different in efficiency, according to the current consumption of a load connected thereto.
However, even according to this conventional technique, the power loss described above can occur if an input voltage margin is allowed, because there is only one power supply system (the direct-current power supply).